1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD), and more particularly, to an LCD capable of removing a residual direct current (DC).
2. Description of the Related Art
An LCD is a flat display for displaying an image by controlling a transmitted amount of a light beam in response to an image signal. Particularly, since the LCD is characteristically lightweight, slim, and has low power consumption, the application field of the LCD is gradually expanding.
The LCD includes a liquid crystal (LC) panel for displaying an image, gate drivers for applying gate signals to the gate lines of the LC panel, and data drivers for applying data signals to the data lines of the LC panel. Currently, the gate drivers and the data drivers operate under control of a timing controller.
The LC panel includes an array substrate having thin film transistors (TFTs) thereon, a color filter substrate having a color filter thereon, and an LC layer interposed between the two substrates.
Referring to FIG. 1, the array substrate includes gate lines 1 arranged in the first direction and data lines 2 arranged in the second direction perpendicular to the first direction and perpendicular to the gate lines. At this point, a pixel region is defined by the gate lines 1 and the data lines 2.
A thin film transistor (TFT) serving as a switching device is connected with each of the gate lines 1 and the data lines 2. That is, a gate electrode of the TFT is connected with each of the gate lines 1 and a source electrode of the TFT is connected with each of the data lines 2. Further, a drain electrode of the TFT is connected with a pixel electrode.
A common electrode can be provided on the array substrate or the color filter substrate depending on the LC mode of the LC layer. That is, in the case of a twisted nematic (TN) mode, the common electrode is provided on the color filter substrate. Accordingly, the common electrode can have a plane shape. In that case, when a predetermined voltage is applied to the pixel electrode and the common electrode, a predetermined vertical electric field is generated between the pixel electrode formed on the array substrate and the common electrode formed on the color filter substrate. The LC is driven by the vertical electric field to display an image.
In the TN mode LCD, the displacement width of the LC is limited and the viewing angle is narrow. On the contrary, in the case of an in-plane switching (IPS) mode, the common electrode is provided on the array substrate together with the pixel electrode. The common electrode can have a line shape and be disposed in parallel with the gate line 1. Pixel electrode bars, which are in parallel with the data lines 2, are arranged in the pixel electrode. Common electrode bars alternately arranged relative to the pixel electrode bars are arranged in the common electrode. In that case, when a predetermined voltage is applied to the pixel electrode and the common electrode, a horizontal electric field (or transverse electric field) is generated between the pixel electrode bars and the common electrode bars. The LC is driven by this horizontal electric field to display an image. In the IPS-mode LCD, the displacement width of the LC is almost not limited and thus the viewing angle is wide.
In the meantime, common electrode lines 3, 4, and 5 for allowing a common voltage provide from the outside to be supplied to the common electrode are arranged along the edge of the LC panel. That is, the first common electrode line 3 is disposed at the left edge of the LC panel, the second common electrode line 4 is disposed at the right edge of the LC panel, and the third common electrode line 5 is disposed at the upper edge of the LC panel. The third common electrode line 5 is connected between the first common electrode line 3 and the second common electrode line 4. A ground line 16 for grounding is disposed at the lower side between the first and second common electrode lines 3 and 4 extending to the lower side of the left/right edge of the LC panel.
In the case where the common electrode lines 3, 4, and 5 are arranged as described above, the gate lines 1 arranged within the LC panel are connected with the first common electrode line 3 with the first static electricity protection pattern 6 interposed. The gate lines 1 would also be connected with the second common electrode line 4 with the second static electricity protection pattern 7 interposed. The third and fourth static electricity protection patterns 8 and 9 are provided to left and right of the third common electrode line 5, respectively. Also, the fifth static electricity protection pattern 10 is provided to each point between the third common electrode line 5 and the data lines 2. Here, the static electricity protection patterns 6, 7, 8, 9, and 10 prevent the inside of the device from being damaged due to electrostatic discharge (ESD). The static electricity protection patterns 6, 7, 8, 9, and 10 can consist of a transistor having a diode function that allows a current to flow in one direction only. That is, the static electricity protection patterns 6, 7, 8, 9, and 10 swiftly emit static electricity generated in the inside of the LC panel to the outside and prevent static electricity generated at the outside from flowing into the inside of the LC panel. Accordingly, thanks to the static electricity protection patterns 6, 7, 8, 9, and 10, the TFT or the pixel electrode provided within the LC panel are safely protected.
In the meantime, when the IPS-mode LCD operates, a residual direct current (DC) appears along the third common electrode line on the upper side and an afterimage or a flicker is generated by this residual DC, which deteriorates image quality.
In the IPS-mode LCD, when the residual DC appears, the residual DC is blocked by the static electricity protection pattern and cannot be emitted to the outside. That is, static electricity protection patterns are formed on the left and the right of the third common electrode line connected between the first and second common electrode lines. When static electricity generated from the outside flows into the LC panel along the first and second common electrode lines, the static electricity protection patterns are conducted to allow the static electricity to flow to the third common electrode line. At this point, since the static electricity protection pattern consists of a diode that allows current to flow in one direction only, the residual DC existing in the inside of the LC panel is blocked by the static electricity protection pattern and cannot be emitted to the outside properly.
The reason the residual DC exists on the third common electrode line has not been clearly examined. Since the residual DC is a primary factor that deteriorates image quality, it is important to remove the residual DC in order to achieve optimum image quality.